- Apheresis - Stem cells will be collected through apheresis, usually starting the 5th to 6th day of Filgrastim injections. For this procedure, whole blood is collected through a needle in an arm vein. The blood circulates through a cell separator machine where the white cells and stem cells are removed. The red cells, platelets and plasma are returned to the body through a second needle in the other arm. The procedure takes about 5 hours. Up to five procedures, done on consecutive days, may be required to collect enough cells for transplantation. If enough cells are collected, they will be purified (treated to remove the white blood cells) using an experimental device. Removing the lymphocytes may reduce the chance of relapse of aplastic anemia following the stem cell transplant. The stem cells will be frozen for later use, if needed.